SPM: Add __ARM_ARCH_8_1M_MAIN__ macro for GNU Arm Embedded Toolchain

Adding __ARM_ARCH_8_1M_MAIN__ compile definition manually for GNU Arm
Embedded Toolchain doesn't emit this macro automatically for the
armv8.1-m.main architecture.

Signed-off-by: Feder Liang <feder.liang@arm.com>
Change-Id: Idce3893cd49ac68364c784f3c7e329752c6aa025
1 file changed