I've uploaded a possible patch to experimental, to avoid introducing new build failures on other archs.
Diff below. diff -Nru acpica-unix-20200925/debian/patches/0001-Add-in-basic-infrastructure-for-big-endian-support.patch acpica-unix-20200925/debian/patches/0001-Add-in-basic-infrastructure-for-big-endian-support.patch --- acpica-unix-20200925/debian/patches/0001-Add-in-basic-infrastructure-for-big-endian-support.patch 2020-10-30 01:15:48.000000000 +0000 +++ acpica-unix-20200925/debian/patches/0001-Add-in-basic-infrastructure-for-big-endian-support.patch 2021-02-16 23:24:40.000000000 +0000 @@ -30,7 +30,7 @@ 9 files changed, 242 insertions(+) create mode 100644 source/components/utilities/utendian.c -Index: acpica-unix2-20200925/generate/unix/acpibin/Makefile +Index: acpica-unix-20200925/generate/unix/acpibin/Makefile =================================================================== --- acpica-unix2-20200925.orig/generate/unix/acpibin/Makefile +++ acpica-unix2-20200925/generate/unix/acpibin/Makefile @@ -106,7 +106,7 @@ =================================================================== --- /dev/null +++ acpica-unix2-20200925/source/components/utilities/utendian.c -@@ -0,0 +1,205 @@ +@@ -0,0 +1,225 @@ +/****************************************************************************** + * + * Module Name: utendian -- byte swapping support for other-endianness @@ -263,7 +263,27 @@ + return Result; +} +#else -+UINT64 AcpiUtReadUint64 (void *SrcPtr) { return *(UINT64 *)SrcPtr; } ++UINT64 AcpiUtReadUint64 (void *SrcPtr) { ++#if defined(ACPI_MISALIGNMENT_NOT_SUPPORTED) ++ UINT64 Result = 0; ++ UINT8 *Dst = (UINT8 *)&Result; ++ UINT8 *Src = (UINT8 *)SrcPtr; ++ ++ Dst[7] = Src[7]; ++ Dst[6] = Src[6]; ++ Dst[5] = Src[5]; ++ Dst[4] = Src[4]; ++ Dst[3] = Src[3]; ++ Dst[2] = Src[2]; ++ Dst[1] = Src[1]; ++ Dst[0] = Src[0]; ++ ++ return Result; ++ ++#else ++ return *(UINT64 *)SrcPtr; ++#endif ++} +#endif + +/******************************************************************************* diff -Nru acpica-unix-20200925/debian/patches/fix_ftbfs_mips64el.patch acpica-unix-20200925/debian/patches/fix_ftbfs_mips64el.patch --- acpica-unix-20200925/debian/patches/fix_ftbfs_mips64el.patch 1970-01-01 00:00:00.000000000 +0000 +++ acpica-unix-20200925/debian/patches/fix_ftbfs_mips64el.patch 2021-02-16 23:24:38.000000000 +0000 @@ -0,0 +1,13 @@ +Index: acpica-unix-20200925/source/include/platform/aclinux.h +=================================================================== +--- acpica-unix-20200925.orig/source/include/platform/aclinux.h ++++ acpica-unix-20200925/source/include/platform/aclinux.h +@@ -217,7 +217,7 @@ + + #if defined(__ia64__) || (defined(__x86_64__) && !defined(__ILP32__)) ||\ + defined(__aarch64__) || defined(__PPC64__) ||\ +- defined(__s390x__) ||\ ++ defined(__s390x__) || defined(__mips64) ||\ + (defined(__riscv) && (defined(__LP64__) || defined(_LP64))) + #define ACPI_MACHINE_WIDTH 64 + #define COMPILER_DEPENDENT_INT64 long diff -Nru acpica-unix-20200925/debian/patches/series acpica-unix-20200925/debian/patches/series --- acpica-unix-20200925/debian/patches/series 2020-10-30 01:15:48.000000000 +0000 +++ acpica-unix-20200925/debian/patches/series 2021-02-16 23:24:29.000000000 +0000 @@ -61,3 +61,4 @@ armv7-str-fixes.patch dbtest.patch ull-32bit.patch +fix_ftbfs_mips64el.patch