Update llvmlibc.md documentation to include AArch64 support and heap. (#521)

Support for AArch64 has been available for AArch64 in llvm-libc for some
time, but the docs still claim this isn't supported.

Also update the instructions for defining the bounds of the heap for
when malloc is used.

Author: smithp35

docs/llvmlibc.md

@@ -47,6 +47,13 @@ following command line options, in addition to `--target`, `-march` or
   the symbol `__stack` in addition to whatever other memory layout you
   want.
 
+* `-Wl,__llvm_libc_heap_limit=0x`_nnnnnn_ if you are using the heap.
+  The heap start defined by the value of the symbol `_end` which will
+  be defined by the linker if no linker script is used. Alternatively
+  use a linker script that defines the symbols `_end` and
+  `__llvm_libc_heap_limit` in addition to whatever other memory layout
+  you want.
+
 For example:
 
 ```
@@ -55,9 +62,6 @@ clang --config=llvmlibc.cfg --target=arm-none-eabi -march=armv7m -o hello hello.
 
 ## Limitations of LLVM libc in LLVM Embedded Toolchain for Arm
 
-At present, this toolchain only builds LLVM libc for AArch32, not for
-AArch64.
-
 At present, this toolchain does not build any C++ libraries to go with
 LLVM libc.