package/gmp: guard riscv definition

In commit 30997eaa65438a2ce726ad8a204ac5a36363f5c8 a mistake
was made. Guard the definition correctly.

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/gmp/gmp.mk

@@ -24,7 +24,7 @@ GMP_CONF_OPTS += --disable-assembly
 endif
 
 # GMP needs M extension for riscv assembly
-ifeq ($(BR2_RISCV_ISA_CUSTOM_RVM),)
+ifeq ($(BR2_riscv):$(BR2_RISCV_ISA_CUSTOM_RVM),y:)
 GMP_CONF_OPTS += --disable-assembly
 endif