ARM: do not add linker flag `-lm` unconditionally

On ARM the required math library depends on whether the soft floating
point ABI is used or not but this is already handled in
`Makefile.system`, lines 499-505.

Makefile.system

@@ -242,10 +242,6 @@ EXTRALIB	+= -lm
 NO_EXPRECISION = 1
 endif
 
-ifeq ($(OSNAME), Android)
-EXTRALIB	+= -lm
-endif
-
 ifeq ($(OSNAME), AIX)
 EXTRALIB	+= -lm
 endif