package/gmp: fix !RISC-V RVM conditional after merge of next

This was added both on master and next, but the symbol was also renamed in
next by commit cbd91e89e43d6 (arch/Config.in.riscv: allow extensions for
generic) and resolved wrong.  Ensure we use the new symbol name.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/gmp/gmp.mk

@@ -21,7 +21,7 @@ GMP_CONF_OPTS += --disable-assembly
 endif
 
 # GMP needs M extension for riscv assembly
-ifeq ($(BR2_riscv):$(BR2_RISCV_ISA_CUSTOM_RVM),y:)
+ifeq ($(BR2_riscv):$(BR2_RISCV_ISA_RVM),y:)
 GMP_CONF_OPTS += --disable-assembly
 endif