Only BL1 specifies '.' in the address field of the stacks section.

Commit 4f59d835 ("Make BL1 RO and RW base addresses configurable")
added '.' on purpose but the commit message does not help to understand
why.

This commit gets rid of it in order to factor out the stacks section
into include/common/bl_common.ld.h

I compared the build result for PLAT=qemu.

'aarch64-linux-gnu-nm -n build/qemu/release/bl1/bl1.elf' will change
as follows:

@@ -336,8 +336,8 @@
 000000000e04e0e0 d max_log_level
 000000000e04e0e4 D console_state
 000000000e04e0e5 D __DATA_RAM_END__
-000000000e04e0e5 B __STACKS_START__
 000000000e04e100 b platform_normal_stacks
+000000000e04e100 B __STACKS_START__
 000000000e04f100 b bl1_cpu_context
 000000000e04f100 B __BSS_START__
 000000000e04f100 B __STACKS_END__

After this change, __STACKS_START__ will match to platform_normal_stacks,
and I think it makes more sense.

'aarch64-linux-gnu-objdump -h build/qemu/release/bl1/bl1.elf' w...