libc: enable initial-exec (IE) as default thread-local storage model …

…on arm

As suggested by jrtc27@ in https://reviews.freebsd.org/D42415, this
patch enables IE as default thread-local storage model in libc on arm.

Reviewed by:	kib
Sponsored by:	Rubicon Communications, LLC ("Netgate")
Differential Revision:	https://reviews.freebsd.org/D42445

lib/libc/Makefile

@@ -54,11 +54,7 @@ CFLAGS+=${CANCELPOINTS_CFLAGS}
 
 # Use a more efficient TLS model for libc since we can reasonably assume that
 # it will be loaded during program startup.
-.if ${LIBC_ARCH} == "aarch64" || ${LIBC_ARCH} == "amd64" || \
-    ${LIBC_ARCH} == "i386" || ${LIBC_ARCH} == "riscv" || \
-    ${LIBC_ARCH:Mpowerpc*} != ""
 CFLAGS+= -ftls-model=initial-exec
-.endif
 
 #
 # Link with static libcompiler_rt.a.