Fix __errno for LP64 and clean up __get_tls.

If __get_tls has the right type, a lot of confusing casting can disappear.

It was probably a mistake that __get_tls was exposed as a function for mips
and x86 (but not arm), so let's (a) ensure that the __get_tls function
always matches the macro, (b) that we have the function for arm too, and
(c) that we don't have the function for any 64-bit architecture.

Change-Id: Ie9cb989b66e2006524ad7733eb6e1a65055463be

libc/Android.mk

@@ -185,6 +185,7 @@ ifeq ($(TARGET_ARCH),$(filter $(TARGET_ARCH),arm mips x86))
 libc_common_src_files += \
     bionic/fcntl.c \
     bionic/fstatfs.c \
+    bionic/__get_tls.cpp \
     bionic/lseek64.c \
     bionic/sigsuspend.c \
     bionic/statfs.c \
@@ -217,7 +218,7 @@ libc_bionic_src_files := \
     bionic/assert.cpp \
     bionic/brk.cpp \
     bionic/dirent.cpp \
-    bionic/__errno.c \
+    bionic/__errno.cpp \
     bionic/eventfd_read.cpp \
     bionic/eventfd_write.cpp \
     bionic/futimens.cpp \