[ada] fix unknown type name 'cpu_set_t'

Hi, When building ada with musl, I encountered the following error: make[7]: Entering directory '/opt/gcc-build/gcc/build/gcc/ada/rts' /opt/gcc-build/gcc/build/./gcc/xgcc -B/opt/gcc-build/gcc/build/./gcc/ -B/opt/gcc-13/aarch64-linux-musl/usr/aarch64-linux-musl/bin/ -B/opt/gcc-1

[PATCH] ada: Fix musl build on Linux

The commit "ada: Add PIE support to backtraces on Linux" [1] use _r_debug under Linux unconditionally. It is incorrect since musl[2] libc not defined _r_debug like glibc [3]: extern struct r_debug _r_debug; As far as I know, only glibc and uClibc [4] define the global variable _r_debug. [1] h