Relaxable TLSDESC only requires that R_RISCV_TLSDESC_HI20 to be paired with R_RISCV_RELAX

Requiring 4 R_RISCV_RELAX relocations impose a large size increase to
the relocatable file. To mitigate this size increase, we can say that
the whole TLSDESC code sequence is relaxable if the first instruction
(R_RISCV_TLSDESC_HI20) is paired with R_RISCV_RELAX.

A statically linked executable typically has a simple relocation
resolver that handles just RELATIVE/IFUNC. For a `-fpic -mtls-dialect=desc`
relocatable file, the linker is required to perform TLS optimization to
local-exec for a statically linked executable. Therefore, instruction
rewriting is essentially inevitable if the relocation resolver is kept
simple. Ths general-dynamic TLS model, without a defined optimization
scheme, actually has the same issue.

Author: MaskRay

riscv-elf.adoc

@@ -1848,9 +1848,9 @@ Relaxation candidate (`tX` and `tY` can be any combination of two general purpos
 ----
 label:
 	auipc tX, <hi>      // R_RISCV_TLSDESC_HI20 (symbol), R_RISCV_RELAX
-	lw    tY, tX, <lo>  // R_RISCV_TLSDESC_LOAD_LO12 (label), R_RISCV_RELAX
-	addi  a0, tX, <lo>  // R_RISCV_TLSDESC_ADD_LO12 (label), R_RISCV_RELAX
-	jalr  t0, tY        // R_RISCV_TLSDESC_CALL (label), R_RISCV_RELAX
+	lw    tY, tX, <lo>  // R_RISCV_TLSDESC_LOAD_LO12 (label)
+	addi  a0, tX, <lo>  // R_RISCV_TLSDESC_ADD_LO12 (label)
+	jalr  t0, tY        // R_RISCV_TLSDESC_CALL (label)
 ----
 
 Relaxation result:
@@ -1889,9 +1889,9 @@ Relaxation candidate (`tX` and `tY` can be any combination of two general purpos
 ----
 label:
 	auipc tX, <hi>      // R_RISCV_TLSDESC_HI20 (symbol), R_RISCV_RELAX
-	lw    tY, tX, <lo>  // R_RISCV_TLSDESC_LOAD_LO12 (label), R_RISCV_RELAX
-	addi  a0, tX, <lo>  // R_RISCV_TLSDESC_ADD_LO12 (label), R_RISCV_RELAX
-	jalr  t0, tY        // R_RISCV_TLSDESC_CALL (label), R_RISCV_RELAX
+	lw    tY, tX, <lo>  // R_RISCV_TLSDESC_LOAD_LO12 (label)
+	addi  a0, tX, <lo>  // R_RISCV_TLSDESC_ADD_LO12 (label)
+	jalr  t0, tY        // R_RISCV_TLSDESC_CALL (label)
 ----
 
 Relaxation result (long form):