linker: Clean up section name definitions

This commit cleans up the section name definitions in the linker
sections header file (`include/linker/sections.h`) to have the uniform
format of `_(SECTION)_SECTION_NAME`.

In addition, the scope of the short section reference aliases (e.g.
`TEXT`, `DATA`, `BSS`) are now limited to the ASM code, as they are
currently used (and intended to be used) only by the ASM code to
specify the target section for functions and variables, and these short
names can cause name conflicts with the symbols used in the C code.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>