riscv: Access gd with inline assembly when building with LTO or Clang

riscv: Access gd with inline assembly when building with LTO or Clang

Similar to AArch64's case, Clang may wrongly fold accesses to gd pointer
which is defined with register qualifier into constants, breaking
various components.

This patch defines gd as a macro when building with Clang or LTO, which
expands to get_gd() that accesses gp pointer in assembly, making RISC-V
ports function properly and preparing for introduction of LTO in the
future. Board initialization code is also adapted for non-assignable gd.

Reported-by: Nathaniel Hourt <I@nathaniel.land>
Signed-off-by: Yao Zi <ziyao@disroot.org>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>