testsuite: Add -mbig-endian stdint.h requirement

Some AArch64 tests force -mbig-endian and indirectly include stdint.h.
However, not all OSes support both endiannesses, leading to:

FAIL: gcc.target/aarch64/advsimd-intrinsics/bf16_get-be.c   -O0  (test for excess errors)
UNRESOLVED: gcc.target/aarch64/advsimd-intrinsics/bf16_get-be.c   -O1   check-function-bodies test_vget_high_bf16
UNRESOLVED: gcc.target/aarch64/advsimd-intrinsics/bf16_get-be.c   -O1   check-function-bodies test_vget_low_bf16

etc.  This patch adds an (admittedly special-purpose) target
selector for this.

gcc/
	* doc/sourcebuild.texi (stdint_types_mbig_endian): Document.

gcc/testsuite/
	* lib/target-supports.exp
	(check_effective_target_stdint_types_mbig_endian): New proc.
	* gcc.target/aarch64/advsimd-intrinsics/bf16_get-be.c: Require
	stdint_types_mbig_endian.
	* gcc.target/aarch64/advsimd-intrinsics/bfdot-2.c: Likewise.
	* gcc.target/aarch64/advsimd-intrinsics/vdot-3-2.c: Likewise.