rst optimization
is to combine identical literal values to reduce code size.  A redundant
literal will be removed and all the 'L32R' instructions that use it will
be changed to reference an identical literal, as long as the location of
the replacement literal is within the offset range of all the 'L32R'
instructions.  The second optimization is to remove unnecessary overhead
from assembler-generated "longcall" sequences of 'L32R'/'CALLXN' when
the target functions are within range of direct 'CALLN' instructions.

   For each of these cases where an indirect call sequence can be
optimized to a direct call, the linker will change the 'CALLXN'
instruction to a 'CALLN' instruction, remove the 'L32R' instruction, and
remove the literal referenced by the 'L32R' instruction if it is not
used for anything else.  Removing the 'L32R' instruction always reduces
code size but can potentially hurt performance by changing the alignment
of subsequent branch targets.  By default, the linker will always
preserve alignments, either by switching some instructions between
24-bit encodings and the equivalent density instructions or by inserting
a no-op in place of the 'L32R' instruction that was removed.  If code
size is more important than performance, the '--size-opt' option can be
used to prevent the linker from widening density instructions or
inserting no-ops, except in a few cases where no-ops are required for
correctness.

   The following Xtensa-specific command-line options can be used to
control the linker:

'--size-opt'
     When optimizing indirect calls to direct calls, optimize for code
     size more than performance.  With this option, the linker will not
     insert no-ops or widen density instructions to preserve branch
     target alignment.  There may still be some cases where no-ops are
     required to preserve the correctness of the code.