testsuite: arm: Use -mcpu=unset when overriding -march

testsuite: arm: Use -mcpu=unset when overriding -march

Update test cases to use -mcpu=unset/-march=unset feature introduced in
r15-3606-g7d6c6a0d15c.

gcc/testsuite/ChangeLog:
	* gcc.dg/pr41574.c: Added option "-mcpu=unset".
	* gcc.dg/pr59418.c: Likewise.
	* lib/target-supports.exp (add_options_for_vect_early_break):
	Likewise.
	(add_options_for_arm_v8_neon): Likewise.
	(check_effective_target_arm_neon_ok_nocache): Likewise.
	(check_effective_target_arm_simd32_ok_nocache): Likewise.
	(check_effective_target_arm_sat_ok_nocache): Likewise.
	(check_effective_target_arm_dsp_ok_nocache): Likewise.
	(check_effective_target_arm_crc_ok_nocache): Likewise.
	(check_effective_target_arm_v8_neon_ok_nocache): Likewise.
	(check_effective_target_arm_v8_1m_mve_fp_ok_nocache): Likewise.
	(check_effective_target_arm_v8_1a_neon_ok_nocache): Likewise.
	(check_effective_target_arm_v8_2a_fp16_scalar_ok_nocache):
	Likewise.
	(check_effective_target_arm_v8_2a_fp16_neon_ok_nocache):
	Likewise.
	(check_effective_target_arm_v8_2a_dotprod_neon_ok_nocache):
	Likewise.
	(check_effective_target_arm_v8_1m_mve_ok_nocache): Likewise.
	(check_effective_target_arm_v8_2a_i8mm_ok_nocache): Likewise.
	(check_effective_target_arm_fp16fml_neon_ok_nocache): Likewise.
	(check_effective_target_arm_v8_2a_bf16_neon_ok_nocache):
	Likewise.
	(check_effective_target_arm_v8m_main_cde_ok_nocache): Likewise.
	(check_effective_target_arm_v8m_main_cde_fp_ok_nocache):
	Likewise.
	(check_effective_target_arm_v8_1m_main_cde_mve_ok_nocache):
	Likewise.
	(check_effective_target_arm_v8_1m_main_cde_mve_fp_ok_nocache):
	Likewise.
	(check_effective_target_arm_v8_3a_complex_neon_ok_nocache):
	Likewise.
	(check_effective_target_arm_v8_3a_fp16_complex_neon_ok_nocache):
	Likewise.
	(check_effective_target_arm_v8_1_lob_ok): Likewise.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>