formula: add rpath helper method

About 40 formulae set `CMAKE_INSTALL_RPATH` to `lib` or `opt_lib`, but
this breaks bottle relocatability.

The correct solution is to use `@loader_path/../lib`, but this is macOS
specific, so it requires some OS-specific logic. Rather than replicating
this logic over many formulae, we may as well define a helper method for
it.

See https://github.com/Homebrew/homebrew-core/issues/75458.

Library/Homebrew/extend/os/linux/formula.rb

@@ -3,6 +3,7 @@
 
 class Formula
   undef shared_library
+  undef rpath
 
   def shared_library(name, version = nil)
     suffix = if version == "*" || (name == "*" && version.blank?)
@@ -13,6 +14,10 @@ class Formula
     "#{name}.so#{suffix}"
   end
 
+  def rpath
+    "'$ORIGIN/../lib'"
+  end
+
   class << self
     undef ignore_missing_libraries
 

Library/Homebrew/formula.rb

@@ -1523,6 +1523,12 @@ class Formula
     "#{name}#{infix}.dylib"
   end
 
+  # Executable/Library RPATH according to platform conventions.
+  sig { returns(String) }
+  def rpath
+    "@loader_path/../lib"
+  end
+
   # an array of all core {Formula} names
   # @private
   def self.core_names