[sysvabi64] No BTI landing pads for absolute symbols.

It is possible to provide a static linker with branch targets
using absolute symbols. For example using --defsym. The static
cannot disassemble the location of these symbols so it isn't
possible to check whether A BTI landing pad is necessary, nor
is it always possible to place a BTI landing pad within range
of the absolute symbol address.

A user providing the static linker with function addresses
using absolute symbols is responsible for putting BTI
compatible landing pads at the start of these addresses.

sysvabi64/sysvabi64.rst

@@ -1706,7 +1706,8 @@ A static linker is required to generate `Custom PLTs`_ with BTI
 instructions.
 
 A static linker that uses indirect branches in veneers is required to
-generate a BTI compatible landing pad if the target does not have a
+generate a BTI compatible landing pad if the target of the indirect
+branch is defined within the same link unit and does not have a
 compatible BTI instruction at the destination of the veneer. A BTI
 compatible landing pad consists of a BTI instruction followed by a
 direct branch. For example:
@@ -1726,6 +1727,9 @@ direct branch. For example:
   fn:
   // a non BTI instruction.
 
+A static linker is not required to insert BTI compatible landing pads
+for symbols with section index ``SHN_ABS``.
+
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