Define contrainsts for ULEB128

Binutils [1] and a currently in-flight LLD patch [2] define ULEB128 such
that a SUB_ULEB128 must be preceded by a matching SET_ULEB128. This
means that if the first SET_ULEB128 that comes before a given
SUB_ULEB128 does not have the same relocation address, the linkers will
fail. A SET_ULEB128 also cannot appear alone.

Define this behavior in psABI such that libraries that read relocations
are able to make this same assumption.

[1] https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=bfd/elfnn-riscv.c;h=09aa7be225ef3cba23fefd2ee977b3556a51368c;hb=HEAD#l2506
[2] https://reviews.llvm.org/D142880

Signed-off-by: Charlie Jenkins <[email protected]>

riscv-elf.adoc

@@ -456,9 +456,9 @@ Description:: Additional information about the relocation
                                             <| `ifunc_resolver(B + A)`
 .2+| 59      .2+| PLT32         .2+| Static  | _word32_          .2+| 32-bit relative offset to a function or its PLT entry
                                             <| S + A - P
-.2+| 60      .2+| SET_ULEB128   .2+| Static  | _ULEB128_         .2+| Local label assignment <<uleb128-note,*note>>
+.2+| 60      .2+| SET_ULEB128   .2+| Static  | _ULEB128_         .2+| Must be paired with, and come before, a SUB_ULEB128. Local label assignment <<uleb128-note,*note>>
                                             <| S + A
-.2+| 61      .2+| SUB_ULEB128   .2+| Static  | _ULEB128_         .2+| Local label subtraction <<uleb128-note,*note>>
+.2+| 61      .2+| SUB_ULEB128   .2+| Static  | _ULEB128_         .2+| Must be paired with the first SET_ULEB128 that comes before this. Local label subtraction <<uleb128-note,*note>>
                                             <| V - S - A
 .2+| 62      .2+| TLSDESC_HI20      .2+| Static  | _U-Type_          .2+| High 20 bits of a 32-bit PC-relative offset into a TLS descriptor entry, `%tlsdesc_hi(symbol)`
                                             <| S + A - P