gdb: The signal definitions of musl and gdb collide

gdb: The signal definitions of musl and gdb collide

This fixes compilation of gdb on arm64.

The kernel defines "struct sigcontext" in asm/sigcontext.h and musl libc
defines it in signal.h, which collides.
Kernel 4.14 misses the definitions of struct user_sve_header so we still
have to use the aarch64-sve-linux-sigcontext.h header file which also
provides that and make sure aarch64-sve-linux-sigcontext.h does not
provide the same headers as the kernel or musl.

Fixes: FS#2040
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>