[gdb/testsuite] Require compilation flags in two gdb.arch/aarch64 test-cases

[gdb/testsuite] Require compilation flags in two gdb.arch/aarch64 test-cases

With test-cases gdb.arch/aarch64-mte-core.exp and gdb.arch/aarch64-pauth.exp I
run into compilation errors due to unsupported compilation flags.

Fix this by requiring the compilation flags, such that I have instead:
...
UNSUPPORTED: gdb.arch/aarch64-mte-core.exp: require failed: \
  have_compile_flag -march=armv8.5-a+memtag
UNSUPPORTED: gdb.arch/aarch64-pauth.exp: require failed: \
  have_compile_flag -mbranch-protection=pac-ret+leaf
...

Tested on aarch64-linux.