malloc: use hidden visibility

When running sandbox, the following phases occur, each with different
malloc implementations or behaviors:

1) Dynamic linker execution, using the dynamic linker's own malloc()
implementation. This is fully functional.

2) After U-Boot's malloc symbol has been hooked into the GOT, but before
any U-Boot code has run. This phase is entirely non-functional, since
U-Boot's gd symbol is NULL and U-Boot's initf_malloc() and
mem_malloc_init() have not been called.

At least on Ubuntu Xenial, the dynamic linker does make both malloc() and
free() calls during this phase. Currently these free() calls crash since
they dereference gd, which is NULL.

U-Boot itself makes no use of malloc() during this phase.

3) U-Boot execution after gd is set and initf_malloc() has been called.
This is fully functional, albeit via a very simple malloc()
implementation.

4) U-Boot execution after mem_malloc_init() has been called. This is fully
functional with a complete malloc() implementation.

Furthermore, if code that called malloc() during phase 1 calls free() in
phase 3 or later, it is likely that heap corruption will occur, since
U-Boot's malloc implementation will assume the pointer is part of its own
heap, although it isn't. I have not actively observed this happening.

To prevent phase 2 from happening, this patch makes all of U-Boot's malloc
library public symbols have hidden visibility. This prevents them from
being hooked into the GOT, so only code in the U-Boot binary itself
actually calls them; any other code will call into the standard C library
malloc(). This also avoids the "furthermore" issue mentioned above.

I have seen references to this GCC pragma in blog posts from 2008, and
RHEL5's ancient gcc appears to accept it fine, so I believe it's quite
safe to use it without checking gcc version.

Cc: Rabin Vincent <rabin@rab.in>
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>